1. Field of the Invention
This invention relates to a method of modifying and otherwise treating the surfaces of rotating, sliding and rocking members such as pistons, cylinders and valves that are used in hydraulic pumps and motors in construction equipment. More particularly, this invention relates to steel sliding members improved in low-friction property and high wear-resisting (anti-seizure) characteristics, as well as to a process for producing such steel sliding members and an apparatus to be used in implementing said process.
2. Prior art
While-many processes of metal surface treatment have been proposed for improving the wear resistance, fatigue resistance, lubrication and anti-seizure characteristics of steel members, sulphonitriding is especially suited to the purpose of improving the sliding characteristics of metals in a "boundary lubrication region" where the sliding condition is so hostile as to potentially cause a local direct contact of two metal parts if the lubricant film is broken.
Sulphonitriding is commonly performed by a "mixed salt bath" method using neutral or reducing salt baths added with sulfur compounds. However, the cyanides or cyanates in the salt baths are prone to degradation or deterioration at elevated temperatures and it is difficult to maintain a uniform salt bath composition for an extended time. Further, the use of very poisonous cyanides is not desirable from the viewpoint of preventing pollutions. The difficulty in controlling the bath concentration makes it difficult to treat many types of steel materials with appropriate layers. Further, the liquid bath components sometimes fail to reach every portion of the steel member being treated and there is a high possibility that some portions remain untreated.
A gas sulphonitriding process has been proposed with a view to solving the aforementioned problems of the mixed salt bath method. According to gas sulphonitriding, steel members of any complex shapes can be sulphonitrided almost completely and few or no portions of the steel member remain untreated. However, an .epsilon.-Fe.sub.2-3 N layer (usually called a "white layer") will form if the surface treatment is conducted by the conventional gas sulphonitriding process. This .epsilon.-Fe.sub.2-3 N layer has a hard and brittle mechanical property and has been unable to achieve satisfactory improvements in the wear resistance of steel members.